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You should limit the numer of species in a game based on how many players are on because whenever i join a mulitplayer macth there is always a overwelming amount of aleins a medium numer of marines and about 1 or 2 predators.It would be much better if there was a limit on the species
Choose to host a match and you can set all those options. Once you leave, the match will continue as you set it with some other player becoming host so feel free to start matches. From my experience they tend to populate quickly, but it may depend on time of day, etc.